SRM conundrum

andrewbradley

In an article on the "benefits of spin" in the cycling press this week the author claims that an SRM will show that using the 16 sprocket at 85rpm requires much less energy than the 15 sprocket at 80rpm (for the same speed on the road/trainer of approx 24mph).

Could it be the SRM that is playing up?
How accurate are these things?

beerco

He is definitely on crack. It would definitely not take "much less" energy. Three things:

1) There may be very very small (on the order of like 2 watts) differences in power requirements between gears due to the drivetrain.

2) In reading the websites of companies which do dynamic calibrations of the SRM, the SRM seems to suffer from some very minor "gear error" i.e. the SRM will give slightly different readings depending on which gear you are in.

and finally and possibly most significantly:

3) If it was an SRM amateur, that could be the problem. They are well known to often not have a linear torque response curve which could lead one to believe what this guy wrote because one of the symptoms of a non linear torque response would be just that - differing power readings at differing cadences.

Also, be weary of any "study" with a sample size of one.

Wanna talk spin? Check this out:

http://www.midweekclub.com/articles/coyle91.pdf

andrewbradley

I was thinking along those lines. The suggestion that the SRM reading confirms improved pedalling efficiency at higher RPM (rather than drivetrain issues) is certainly inspired even by infomercial standards.



Sounds likely.

Unfortunately the article will only serve to reinforce the myth that higher cadence is more efficient and that you can never pedal fast enough.

2LAP

Surely oxygen consumption is required to make this calculation. Simularly, this would be different between different riders as pedaling economy (energy cost) is different between riders, cadences and power outputs.

I think that Ric will confirm on his return, but I think that lower cadences are more efficient than high cadences at any given power outout. This would be reasonable given that more 'internal' work is performed the faster the legs go. This is demonstrated by a greater oxygen consumption, the faster the legs move in conditions where there is no resistance.

The most efficent cadence will not be found at the highest cadences or lowest cadences; rather in the middle. This will be different for everyone.

Out on the road gear choice will rarely be selected against the most 'efficent' cademce for a given output. Rather selection of the optimum gear depends upon other factors, like tactics, fatigue, etc.

It is likely that experianced cyclists who are free to choose their gears will chose the optimum gear for any give situation, and this selection becomes more refined with increased experiance. A similar situation exists for runners whereby they choose the optimum stride frequency and length at any given speed; walkers do the same thing when walking.

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andrewbradley

You would certainly have thought so. You being in Britain did you read the article? Some gems in there.

This is the case from the data I've seen.

2LAP

Missed the article, which mag is it in?

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andrewbradley

The Comic

2LAP

I'll try to check it out.

Thanks

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ric_stern/RST

I'm back!!! Yes, lower cadences are more efficient (which can seem paradoxical with all the talk of fast cadences). As the absoloute workload increases (i.e. power increases) so does the the most efficient cadence (e.g. at 200 W the most efficient cadence maybe 60 revs/min, at 400 W it maybe 80 revs/min -- note these numbers aren't necessarily correct).

However, and importantly, the most efficient cadence isn't necessarily the most optimal cadence, additionally, racing isn't necessarily about being efficient either (but being able to put out the best scaled power at crucial points).

Ric

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